1. Field of the Invention
The present invention relates to a ferroelectric thin film containing lead (Pb), a method of producing said thin film and a coating solution for making the ferroelectric thin film.
2. Description of the Related Art
A ferroelectric thin film has received attention in recent years, since it is expected that the ferroelectric thin film can be applied to the production of semiconductor memory devices and the like. (For example, attention is directed to literature I: Ceramics Vol. 30, No. 6, pp. 499-507 (1995)). In this respect, studies on a ferroelectric thin film of a composition of PbZrxTi 1xe2x88x92xO3(0 less than x less than 1, hereinafter referred to as xe2x80x9cPZTxe2x80x9d) have been particularly carried out. For making the PZT thin film, several methods, such as a sputtering method, a CVD method, a wet method or the like, have been tried. Since the wet method does not use a vacuum system, it is superior in the points that it can reduce the cost of making the thin film and mass-producing it and it is capable of producing a thin film on a large area. According to the wet method, a coating solution for making the ferroelectric thin film is applied on a substrate and then is baked to produce the desired thin film. A coating solution for making the PZT thin film is manufactured by a so-called sol-gel method in many cases.
There is a problem, however, in that a large leakage current tends to flow through the prior art PZT thin film. In particular, there is a problem in the thinner film thickness range (i.e. in a range of not more than 0.3 xcexcm) in that the dielectric withstand voltage of the film deteriorates in addition to the increased leakage current. These problems are caused by the lead (Pb) loss in the thin film. Moreover, the PZT film made by the wet method in particular tends to possess these problems. This is thought to be due to the occurrence of lead loss in the baking process. To solve this problem, there has been proposed a method of adding excessive lead in advance in amounts equivalent to the loss (a few %). But even in this case, the thin film is in such a state that it is deficient in lead in close proximity to the surface thereof. Further, if lead is added in excessive amounts to avoid this state of lead deficiency, the electrical conductivity of the film is increased; thus, the leakage current is increased. It is thought that not only PZT thin films but also other various kinds of ferroelectric thin films containing Pb will produce this problem.
Accordingly, it is desirable to produce a ferroelectric thin film containing Pb and with a smaller leakage current than was previously possible and a process of making such film, as well as a coating solution for producing the thin film.
With a view to correcting the above deficiencies there is provided, according to a first aspect of the present invention, a ferroelectric thin film containing Pb, comprising antimony (Sb) present in sufficient quantities to compensate for a change of the thin film into a p-type film caused by the Pb defects generated in the thin film. If such a quantity of Sb is added in the ferroelectric thin film containing Pb, even if bivalent Pb is dispersed during the process of manufacturing the ferroelectric thin film, thus producing Pb defects in the thin film, the trivalent Sb compensates for such Pb defects. The ferroelectric thin film might slightly become a p-type film, even with the addition of Sb, however, it maintains its insulation properties, thereby restraining the occurrence of leakage current.
In accordance with a second aspect of the present invention, there is provided a method for making a ferroelectric thin film containing lead on a substrate by a wet method, which method comprises the steps of:
adding in advance in a coating solution used for making a ferroelectric thin film, antimony (Sb) in such quantities as to compensate for a change of the thin film into p-type that is caused by Pb defects generated in the thin film, and
forming a ferroelectric thin film containing lead on a substrate by using a coating solution containing antimony.
According to the foregoing method, the production of a ferroelectric thin film by a wet method is facilitated and the cost of making such film is significantly reduced. This is achieved by adding Sb in advance to the coating solution to produce a ferroelectric thin film containing Pb.
In accordance with a third aspect of the present invention, there is provided a coating solution for making a ferroelectric thin film containing Pb by a wet method, comprising antimony in a range of 0.01-5 mol % based upon 100 mol % of Pb. The reason why the ratio of Sb to Pb is in a range of 0.01-5 mol % is that the addition of Sb in this range reduces the leakage current without segregation of the Sb.
The foregoing and other objects, features and advantages of the present invention will be better understood from the following description taken in connection with the accompanying drawings.